A sun energy collecting unit (solar collector) of a sun field power plant based on the concentrated solar power technique is for instance a solar collector with a parabolic mirror and a heat receiver tube. The heat receiver tube is arranged in a focal line of a solar radiation (sunlight) reflecting surface of the mirror. By the solar radiation reflecting surface sunlight is collected and focused to the heat receiver tube.
The heat receiver tube comprises a core tube (inner tube, e.g. made of stainless steel) which is filled with a heat transfer fluid, e.g. a thermo-oil or molten salt. With the aid of a solar radiation absorptive coating of the core tube the heat receiver tube absorbs energy from the sun. Energy from the sun is efficiently coupled into the heat transfer fluid. Solar energy is converted into thermal energy.
In order to minimize a loss of thermal energy, the heat receiver tube comprises an encapsulation with an enveloping tube. The enveloping tube envelops the core tube.
For instance, the enveloping tube is a glass tube. This enveloping tube is at least partly transparent for solar radiation. So, solar radiation can impinge the solar radiation absorptive coating of the core tube.
The core tube and the enveloping tube are coaxially arranged to each other resulting in an inner space of the heat receiver tube which is bordered by a core tube surface of the core tube and by an inner enveloping tube surface of an enveloping tube wall of the enveloping tube.
The inner space of the heat receiver tube between the inner tube and the enveloping tube is evacuated in order to minimize convection and hence in order to minimize a thermal loss of the heat receiver tube. The inner heat receiver tube space is a vacuum chamber.
One problem is a degradation of the heat transfer fluid during operation for years. By the degradation Hydrogen (H2) is formed. This Hydrogen permeates through the stainless steel wall of the core tube into the evacuated inner space of the heat transfer fluid. The result is a collapse of the vacuum of the inners space of the heat receiver tube and hence an increase of the thermal loss of the heat receiver tube.